quic: Avoid possible buffer overflow in find_bucket

Proved by fuzzing the code.
Signed-off-by: Frediano Ziglio <freddy77@gmail.com>
Acked-by: Uri Lublin <uril@redhat.com>

common/quic_family_tmpl.c

@@ -103,7 +103,12 @@ static s_bucket *FNAME(find_bucket)(Channel *channel, const unsigned int val)
 {
     spice_extra_assert(val < (0x1U << BPC));
 
-    return channel->_buckets_ptrs[val];
+    /* The and (&) here is to avoid buffer overflows in case of garbage or malicious
+     * attempts. Is much faster then using comparisons and save us from such situations.
+     * Note that on normal build the check above won't be compiled as this code path
+     * is pretty hot and would cause speed regressions.
+     */
+    return channel->_buckets_ptrs[val & ((1U << BPC) - 1)];
 }
 
 #undef FNAME